The present invention relates to an air-flow detecting apparatus for measuring and detecting the amount of intake air of a car.
To determine the amount of fuel to be injected in an internal combustion engine of a car, for example, it is necessary to measure and detect engine rotational speed, temperature of the gas exhausted, temperature of the cooling water, the amount of air taken in, and the like. Signals from sensors which measure these factors are used as operational control data to determine the amount of fuel to be injected. As a typical air-flow detecting sensor, the well-known hot-wire type sensor is disclosed in the U.S. Pat. No. 4,357,830, Japanese Patent Disclosure No. 56/18721, and so on.
FIG. 1 shows an example of an arrangement relating to an air-flow detecting sensor 10 of the hot-wire type, which includes a support frame 11 which is provided with a pair of legs 111 and 112, a heating line 12 wound around the legs 111 and 112 of the frame 11, and a resistance line 13 also wound around the legs 111 and 112 and separated from the heating line 12. The resistance value of the resistance line 13 changes with changes in its temperature. The sensor 10 thus constructed is arranged in the air intake pipe for the car's engine, for example, with the heating line 12 located on the upper side when viewed from the direction G in which the air flows.
Heat emitted from the heating line 12 is transmitted to the resistance line 13 and is detected as a change in the resistance value of the resistance line 13. The path in which the heat is transmitted to the resistance line 13 follows air-flow G. The amount of this air-flow G is therefore measured and calculated from a value which represents the amount of heat transmitted.
With an air-flow detecting sensor 10 like this, however, heat is transmitted from the heating line 12 to the resistance line 13 not only through the air-flow G but also through the legs 111 and 112 of the support frame 11. Therefore, a signal which represents the sum of the amount of heat transmitted both through the air-flow G and through the legs 111 and 112 is supplied from the sensor 10 as a detection signal for the air-flow G.
Assuming that the air-flow G increases as shown by the broken line in FIG. 2, the detection signal from the sensor 10 also changes in correspondence with this air-flow. However, the necessary change in the detection signal follows the change in the amount of the air-flow as shown by the solid line in FIG. 2.
The delayed response at step 1, which is denoted by A, is caused mainly by the heat capacities of the heating and resistance lines 12 and 13 and is relatively small. However, the delayed response at step 2, which is represented by B and which is caused by the heat capacity of the legs 111 and 112 of the support frame 11, will be a long one. This delay B ranges from several seconds to several tens of seconds. The error because of this delay sometimes amounts to 10%.
The length of the delay controls the amount of air intake for the engine. That is, when the detection signal from the sensor 10 is supplied, as it is, to the electronic engine control means to control the amount of fuel injected and the ignition timing, the emission level is remarkably reduced and the overall performance of the engine is improved.